What one is ultimately looking for is neutral pH for anything that touches a paper (or textile) artifact in framing or (theoretically) storage - to the extent possible. Acidic matter touching artifacts, which themselves may be acidic (for example, newsprint), is bad news for longevity of the object.

It is certainly possible to "go crazy" overboard with this issue, but safe to say that anything acidic deteriorates itself and anything it touches, sometimes with surprising rapidity.

A "good" rating would, to me, signify that somewhere along the line of production less-than ideal materials were used in manufacture. I would not lose sleep about it unless we were framing the Magna Carta or something. Something neutral pH is far better than anything acidic.

The term "archival" is pretty much a euphemism for neutral pH - six of one half dozen of another.

I made my living as an artist for 40 years and did some restoration work along the way. Below is a link to where many museum curators go for restoration supplies. The best mat boards have a cotton fiber core and face papers that are acid free (buffered to a pH of 8.5) and lignin free. Cheaper "archival" boards may have acid free cotton cores that are lignin free, but wood pulp face papers that have been acid neutralized. Acid neutralized wood pulp face papers that contain lignin will eventually become acidic - not good. Bainbridge Alpharag mat board would do the trick. So will Crescent RagMat. The better framers will stock it, or can easily get it. Hope this helps.

Having worked professionally as a curator in art museums, and with visual art and artists, for going on 50 years - there is sometimes a tendency to apply in-perpetuity-museum-standards to private or personal preservation measures which is way out of proportion with anything the artifacts/objects truly require.

And it gets really really really expensive, really fast... so much so that even museums are not 100% able to maintain the perfect state of preservation controls for every artifact they protect. There is a kind of triage, so to speak, that comes into play when deciding how much to do for/with each artifact, based on its significance, without intentionally or negligently doing or allowing harm.

Sensible precautions are always in order for any collector or collection. For the average collector, framing or matting something at a place like Michael's (etc) using words such as "archival" or "acid-free" or "neutral pH" will suffice... but chances are the person helping at the counter will not have a clue what the words "lignin-free" relate to.

Suppliers like Talas, as mentioned above, or Gaylord, or University Products, or Hollinger always have the highest standard of product of course. They also offer many choices of ways to put neutral pH barriers into framing, matting or long-term storage projects. Yet, at this point in time, even Target, Amazon and office suppliers like Staples or Office Depot claim to offer what is probably more-than-less archival materials which are perfectly acceptable in most normal applications.